Pathname expansion (i.e. globbing) can't even be used this way as POSIX requires matched names to be lexicographically sorted. That's the first thing that came to mind, before I remembered that brace expansion is a widely-supported extension not defined by POSIX.
Brace expansion doesn't fall under pathname expansion. There is no expectation that the resulting expansion will correspond to files. From `man bash`:
> Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.
That's not to say it's POSIX-compliant--I have no idea whether it is. But it definitely isn't grouped in with pathname expansion. bash does have an option to disable brace expansion, but it's not toggled by `--posix`, which leads me to believe it might be POSIX-compliant.
> Brace expansion doesn't fall under pathname expansion.
I thought that was implied in my post, but perhaps I should have made it explicit.
> That's not to say it's POSIX-compliant--I have no idea whether it is
I didn't know one way or another, but since you brought it up, it looks like it probably is not compliant. See, e.g., this argument that `echo {1,2}` must print '{1,2}' because POSIX doesn't require '{1,2}' to be quoted: https://www.austingroupbugs.net/view.php?id=1193
Brace expansion is a nearly universally supported extension, though, so I doubt it's a real problem. And the above link proposes fixing the standard to make the extension compliant.
